The half-life period of a first order reaction is 15 minutes. The amount of substance left after one hour will be :

(a) $\;\large\frac{1}{4}\;$ of the original amount\[\](b) $\;\large\frac{1}{8}\;$ of the original amount\[\](c) $\;\large\frac{1}{16}\;$ of the original amount\[\](d) $\;\large\frac{1}{32}\;$ of the original amount